1. Field of the Invention
The invention relates to electrochemical reactions and electrodes used therein. More particularly, the invention relates to electrodes for use in the electrochemical removal of nitrates from water, methods for making such electrodes, and apparatus incorporating such electrodes.
2. State of the Art
Parent applications Ser. No. 08/671,264 and Ser. No. 08/457,040 disclose methods and apparatus for the removal of nitrates from water. Although not limited thereto, the apparatus generally includes an electrochemical flow cell through which the aqueous solution containing nitrates flows or a holding tank cell into which the solution is introduced and then released after processing, and an electrode system including a carbon fiber cathodic electrode, a carbon fiber anodic electrode and a reference electrode. All of the electrodes are immersed in the aqueous solution and coupled to an electronic control circuit which impresses a voltage across the electrodes such that the voltage causes electrochemical reduction/oxidation reactions on the surfaces of the cathodic and anodic electrodes. According to the method, the electrodes are at a potential wherein nitrates are reduced to gaseous products but hydrogen, oxygen, chlorine, and other noxious substances are not produced. According to the disclosed preferred embodiment, the reference electrode is a silver/silver-chloride electrode, the cathodic and anodic electrodes are carbon fibers based on polyacrylonitrile (PAN), and the surface area ratio of the anodic electrode to the cathodic electrode is preferably in the range of 40:1 to 120:1.
As disclosed in the parent applications, the anodic to cathodic surface area ratio must be large in order to prevent a chlorine evolution reaction from taking place in salt water and to prevent oxygen evolution reactions and changes in pH in fresh water. In practice, it has been discovered that, in seawater, an anodic to cathodic surface area ratio of up to 150:1 is desirable to prevent chlorine formation under any circumstances. In addition, the anodic voltage (relative to the reference electrode) must be kept below +800 mV to prevent chlorine formation in seawater. Moreover, in flow-through systems, an increased flow rate (flow velocity relative to the cathode) increases the nitrate reduction current and, as a consequence, causes an increase in the anodic voltage.